Particle-specific sorption/desorption properties determine test compound fate and bioavailability in toxicity tests...

We studied the sorption (batch equilibrium experiments) and desorption (consecutively harsher supercritical fluid extractions) of lindane to different types of sediment and food particles, as well as larval uptake in standardized peat-based artificial sediment toxicity tests with the midge Chironomus riparius. Lindane sorption to organic particles was fast and efficient, reaching 98+/-0.1 and 97+/-0.1% of added compound in 48 h for peat and Tetraphyll(R), respectively, and 77+/-0.2% in whole sediment. Sorption to inorganic particles, that is, sand and kaolin clay, was much lower, 9.6+/-1.3% and 8.3+/-0.8%, respectively. Supercritical fluid extractions showed that most of the lindane sorbed to organic particles and sediment was loosely bound, as only 9 to 14% remained associated with particles after weak and intermediate extractions strengths. Larval uptake of dissolved lindane was 4.9+/-0.71 and 10.8+/-1.2 microg/g wet weight in 22 and 68 microg/L treatments, respectively, and four to five times higher than that of particle-associated lindane, ranging 1.0+/-0.15 to 2.7+/-0.21 microg/g in the above treatments. Surprisingly, larval uptake of lindane was similar from refractory peat and the more labile Tetraphyll particles. Despite an efficient larval uptake of dissolved lindane, sorption/desorption of lindane to/from Tetraphyll particles will facilitate digestive uptake in toxicity tests, particularly in spiked-water scenarios where food particles may act as vectors. Our results show that the exposure scenario is an important determinant for the behavior and bioavailability of test compounds in standardized toxicity tests.